drm/xe: improve Kconfig.profile help text for scheduler timeouts

drm/xe: improve Kconfig.profile help text for scheduler timeouts

The existing help texts for the JOB_TIMEOUT, TIMESLICE and PREEMPT_TIMEOUT
configs were brief and did not make the role of each symbol clear:

 - _MIN / _MAX: hard bounds on the per-engine-class timeout. They are
                enforced unconditionally by the sysfs knobs, and (for
                TIMESLICE, the only one exposed via the SET_PROPERTY UAPI)
                they also bound CAP_SYS_NICE requests when
                DRM_XE_ENABLE_SCHEDTIMEOUT_LIMIT is enabled.
 - PREEMPT_TIMEOUT: the boot-time default; the JOB_TIMEOUT and TIMESLICE
                    defaults are hardcoded in the driver, not configured
                    here.

Rewrite the help texts to reflect this, naming the relevant sysfs knobs
and UAPI property explicitly.

v2: Adjusted commit message based on Sashiko's review.

Assisted-by: GitHub-Copilot:claude-sonnet-4.6
Assisted-by: GitHub-Copilot:claude-opus-4.8 #v2
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patch.msgid.link/20260605140951.958172-2-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>